Alkylation is a reaction in which an alkyl group is added to an organic molecule. For example, an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. In petroleum refining, the process reacts a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst to produce an upgraded product stream referred to as alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but because it is free of aromatic components.
Industrial alkylation processes have historically used concentrated hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. As used herein, the term "concentrated hydrofluoric acid" refers to an essentially anhydrous liquid containing at least about 85 weight percent HF.
Hydrofluoric and sulfuric acid alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Hydrogen fluoride, or hydrofluoric acid (HF) is highly toxic and corrosive. However, it is used as a catalyst in isomerization, condensation, polymerization and hydrolysis reactions. The petroleum industry uses anhydrous hydrogen fluoride primarily as a liquid catalyst for alkylation of olefinic hydrocarbons to produce alkylate for increasing the octane number of gasoline. Years of experience in its manufacture and use have shown that HF can be handled safely, provided the hazards are recognized and precautions taken. Though many safety precautions are taken to prevent leaks, massive or catastrophic leaks are feared primarily because the anhydrous acid will fume on escape creating a vapor cloud that can be spread for some distance. Previous workers in this field approached this problem from the standpoint of containing or neutralizing the HF cloud after its release.
U.S. Pat. Nos. 4,938,935 and 4,985,220 to Audeh and Greco, as well as U.S. Pat. No. 4,938,936 to Yan teach various methods for containing and/or neutralizing HF acid clouds following accidental releases.
U.S. Pat. No. 3,795,712 to Torck et al. relates to acid catalysts comprising a Lewis acid, a Bronsted acid, and a sulfone of the formula R--SO.sub.2 --R', where R and R' are each separately a monovalent radical containing from 1 to 8 carbon atoms or form together a divalent radical having from 3 to 12 carbon atoms.
U.S. Pat. No. 3,856,764 to Throckmorton et al. teaches an olefin polymerization catalyst comprising (1) at least one organoaluminum compound, (2) at least one nickel compound selected from the class consisting of nickel salts of carboxylic acids, organic complex compounds of nickel, or nickel tetracarbonyl and (3) at least one hydrogen fluoride complex prepared by complexing hydrogen fluoride with a member of the class consisting of ketones, ethers, esters, alcohols, nitriles, and water.
U.S. Pat. Nos. 4,025,577 and 4,099,924 to Siskin et al. report the use of alkylation catalyst compositions containing HF, a metal halide, and sulfolane. U.S. Pat. No. 5,073,674 to Olah relates to an additive formulation which reduces the fuming tendency of HF.
Promoters such as alcohols, thiols, water, ethers, thioethers, sulfonic acids, and carboxylic acids are disclosed in combination with strong Bronsted acids such as HF, fluorosulfonic and trihalomethanesulfonic acids in U.S. Pat. No. 3,778,489 to Parker et al. The promoters are said to modify the activity of the strong Bronsted acids for alkylation.
The preceding references demonstrate the desirability of a liquid Bronsted acid catalyst (such as HF) for isoparaffin-olefin alkylation, as well as the utility of liquid Bronsted acids in combination with metal halides, particularly metal fluorides.
U.S. Pat. No. 5,191,150 to Child et al. teaches an isoparaffin-olefin alkylation process which addresses the problem of purifying mixtures of HF, sulfolane, and the conjunct polymers (also referred to as acid soluble oil or ASO) formed as byproducts in the HF-catalyzed alkylation of isoparaffins with olefins. The mixture of HF, sulfolane, and ASO (as described in the '150 patent) is difficult to separate because the ASO is a complex mixture of components which boils in a broad range bracketing the boiling point of sulfolane. The entire text of U.S. Pat. No. 5,191,150 is incorporated by reference as if set forth at length herein. For this reason, conventional distillation processes are essentially ineffective for removing ASO from the mixture of HF, sulfolane, and ASO. The process disclosed in the '150 patent first strips the mixture to an HF concentration of 30 weight percent or less, and then gravitationally separates the resulting mixture into an ASO-enriched stream and a sulfolane-enriched stream. Once the HF is stripped from the mixture, however, the process must store and transfer an HF stream containing little or no sulfolane. Thus it would be desirable to provide a method for regenerating an acid catalyst containing HF and sulfolane without stripping the HF from the mixture.